Mass spectrometry is an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three groups of steps—formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means exist in the field of mass spectrometry to perform each of these three functions. The particular combination of means used in a given spectrometer determine the various characteristics of that spectrometer.
To perform mass analysis of ions, for example, one might use a magnetic (B) or electrostatic (E) analyzer, and specifically a combination of both. Ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron impact (EI) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g. semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the sample—e.g. the molecular weight of sample molecules—will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules, however, unlike SIMS, the PD process results also in the desorption of larger, more labile species—e.g. insulin and other protein molecules.
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R. J., Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299). In the MALDI process, analyte is dissolve in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by analyte is used to excite the sample. The matrix is excited directly by the laser. The excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. MALDI is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
It is also known in the prior art to utilize ultrasonic transducers to break up a liquid sample jet into liquid droplets. For example, Miyagi et al., U.S. Pat. No. 4,112,297, disclose a nebulizer which includes an ultrasonic transducer used to create the particle beam. Melera et al., U.S. Pat. No. 4,403,147, incorporate an acoustic transducer, such as a piezoelectric transducer which may be used to stimulate the probe to break up the liquid stream.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
ESMS was introduced by Yamashita and Fenn (M. Yamashita and J. B. Fenn, J. Phys. Chem. 88, 4671, 1984). To establish this combination of ESI and MS, ions had to be formed at atmospheric pressure, and then introduced into the vacuum system of a mass analyzer via a differentially pumped interface. The combination of ESI and MS afforded scientists the opportunity to mass analyze a wide range of samples. ESMS is now widely used primarily in the analysis of biomolecules (e.g. proteins) and complex organic molecules.
In the intervening years a number of means and methods useful to ESMS and API-MS have been developed. Much work has been centered on sprayers and ionization chambers.
For example, Tomany, et al. U.S. Pat. No. 5,304,798 converts an electrospray received from an electrospray apparatus into an ion stream of ions, vapor and gas via a certain housing configuration. The ion stream may be directed through a skimmer, a separate pressure reduction stage and into an analytical apparatus capable of measuring the mass-to-charge spectrum of the sample.
In addition to the original electrospray technique, pneumatic assisted electrospray, dual electrospray, and nano electrospray are now also widely available. Pneumatic assisted electrospray (A. P. Bruins, T. R. Covey, and J. D. Henion, Anal. Chem. 59, 2642,1987) uses nebulizing gas flowing past the tip of the spray needle to assist in the formation of droplets. The nebulization gas assists in the formation of the spray and thereby makes the operation of the ESI easier. Nano electrospray (M. S. Wilm, M. Mann, Int. J. Mass Spectrom. Ion Processes 136, 167, 1994) employs a much smaller diameter needle than the original electrospray. As a result the flow rate of sample to the tip is lower and the droplets in the spray are finer. However, the ion signal provided by nano electrospray in conjunction with MS is essentially the same as with the original electrospray. Nano electrospray is therefore much more sensitive, with respect to the amount of material necessary to perform a given analysis.
The design of the ionization chamber used in conjunction with API-MS has had a significant impact on the availability and use of these ionization methods with MS.
Various apparatuses have been proposed to improve efficiency in mass spectrometry by reducing or controlling the ion flow from the ionization chamber which in turn improves the quality of the interaction between the sample and the mass detection apparatus within the vacuum system. For example, Jarrell, et al. U.S. Pat. Nos. 5,306,910 and 5,436,446 use a time modulated electrospray by the application of a time modulated voltage to an element positioned opposite the electrospray means and analyzer. This is said to reduce sample waste and maintain a low electric potential.
Apffel et al. U.S. Pat. Nos. 5,495,108 and 5,750,988 present apparatuses which increases the enrichment of the analyte entering the vacuum. This is apparently achieved through orthogonal ion sampling whereby charged droplets are sprayed past a sampling orifice while directing the solvent vapor and solvated droplets in a direction such that they do not enter the vacuum system.
Hanson U.S. Pat. No. 5,030,826 discusses an apparatus which redirects vapor spray residue into a coaxial flow system in order to eliminate the necessity for a separate ion outlet port. This is said to simplify maintenance and facilitate vacuum sealing of the components.
Another example of quality control improvements in ionization chambers is discussed in Bertsch, et al. U.S. Pat. No. 5,736,741. Cleaning, maintenance and inspection are facilitated by providing a capillary assembly which may be removed without tools. Bertsch et al. also disclose improvement in the electrical stability of the electrospray ionization chamber by providing an asymmetric electrode. The asymmetric electrode configuration is said to prevent unevaporated droplets and condensation from being trapped, thereby minimizing the chances of electrical breakdown, shorting, arcing or distortion.
Prior art ionization chambers are inflexible to the extent that a given ionization chamber can be used readily with only a single ionization method and a fixed configuration of sprayers. For example, in order to change from a simple electrospray method to a nano electrospray method of ionization, one had to remove the electrospray ionization chamber from the source and replace it with a nano electrospray chamber (see also, Gourley et al., Angled Chamber Seal for Atmospheric Pressure Ionization Mass Spectrometry, U.S. Pat. No. 5,753,910). Thus, a need exists for an ionization chamber which maximizes flexibility and efficiency of use as between various types of samples and analytical methods.